It has been known that biobased polymer products provide sustainability gains through a reduced dependence on petroleum reserves and if the products are biodegradable they also provide environmental amelioration through increased disposal options and lower levels of greenhouse gases [Chiellini and Solaro, 2003; Wool and Sun, 2005]. Low-cost biodegradable plastics and composites are especially sought for high volume applications where large amounts of material are discarded soon after use, as is the case with many types of packaging and some consumer products.
Expanded, cellular products (foams) make up one segment of packaging materials. Foams are used as a protective packaging material for shipping products; the material may be either of the loose-fill type or shaped. With low density packaging material, less packaging weight is needed, reducing both manufacturing and shipping costs. Expanded polystyrene foam, a commonly used packaging material, has the desirable properties of low density, high resiliency, and good moisture and water resistance. Foamed polystyrene, however, is produced from non-renewable, petroleum-based feedstocks. Moreover, it is not biodegradable, which presents a disposal challenge for the large volume of packaging foam that is discarded, typically into landfills and usually soon after use.
Starch-based materials have been of interest because of the generally low cost of starch, and because thermoplastic starch (Avérous, 2004) can be processed with conventional means such as extrusion and injection molding. Starch is both biobased and biodegradable. Various approaches have been used to produce extruded starch foams with properties required for packaging applications. These approaches include the use of high-amylose (45-70%) starch, chemically modified starch, and/or polymer additives.
U.S. Pat. No. 5,208,267, for example, reports the use of blends of normal or high-amylose starch with polyglycols. U.S. Pat. No. 5,272,181 describes extruded foams based on graft copolymers of starch with methyl acrylate. Shogren (1996) reports the extrusion of high-amylose starch acetate foams. U.S. Pat. No. 5,756,556 used chemically modified high-amylose starch, alone or blended with other polymers. U.S. Pat. No. 5,801,207 describes foams based on blends of starch, including chemically modified high-amylose starch, with various polymers, including poly(vinyl alcohol). U.S. Pat. No. 6,107,371 describes the use of chemically modified high-amylose starch with polymer additives including poly(vinyl alcohol). Fang and Hanna (2001) prepared foams using blends of starch and commercial Mater-Bi®. U.S. Pat. No. 6,365,079 describes the extrusion of starch with hydroxy-functionalized polyetheramine. Xu and Hanna (2005) extruded acetylated high-amylose starch foams using water or ethanol. Guan et al. (2005) prepared foams using acetylated native corn starch, high-amylose corn starch or potato starch, blended with polylactic acid and extruded with ethanol. Nabar et al. (2006a) used blends of high-amylose starch and poly(hydroxyl aminoether).
Some packaging applications require foams to be moisture and water resistant, as when products are shipped in humid climates. Foams prepared with chemically unmodified starch and without additives are not suitable for packaging materials where water/moisture resistance is a required property. Various approaches have been used to produce extruded biodegradable and water resistant foams. These approaches include the use of chemically modified starch and/or additives. (U.S. Pat. Nos. 5,208,267; 5,272,181; 5,756,556; 5,801,207; 6,107,371; 6,365,079) A drawback of using chemically modified starch is the added cost.
U.S. Pat. No. 4,863,655, for example, describes the extrusion of high-amylose starch, modified or unmodified, with or without the addition of poly(vinyl alcohol) to produce a biodegradable low-density foam packaging material, but U.S. Pat. No. 5,043,196 (a continuation-in-part of U.S. Pat. No. 4,863,655) reveals that the invention described therein has poor water resistance and disintegrates in water in a matter of minutes.
U.S. Pat. No. 5,554,660 uses blends of high-amylose starch and starch esters to produce water resistant foams. U.S. Pat. No. 5,854,345 describes water resistant foams made from blends of starch with hydroxy functional polyesters. U.S. Pat. No. 6,184,261 blends high-amylose starch with other biodegradable polymers, including poly(lactic acid), to increase the water resistance of foamed materials. Willett and Shogren (2002) measured the water resistance of foams made from normal corn starch blended with other polymers. Guan and Hanna (2004b) report the water resistance of blends of high-amylose starch with starch acetate. Xu et al. (2005) describe the reduced water solubility of starch acetate foams. Nabar et al. (2006b) blended starch with various water-resistant biodegradable polymers to improve the hydrophobic character of the foams. Zhang and Sun (2007) measured the effect on water resistance of blending starch with polylactic acid. Arif et al. (2007) describe the properties, including water resistance, of commercial Green Cell® starch based foam, and Sjöqvist et al. (2010) report the extrusion, water resistance, and other properties of various potato starch foams.
Resistance to water and moisture absorption is only one desirable property in packaging applications. Other desirable properties include low density, high resilience, and high compressive strength.
Starch foams can also be produced with a technique similar to compression molding, whereby a mixture of starch, water, and additives is deposited into heated molds (Tiefenbacher, 1993). Excess water is vented as steam as the mixture expands and fills the mold cavity. A small amount of the mixture tends to be forced through the vents, which builds pressure inside the mold and produces foaming (Tiefenbacher. 1993). The properties of these “baked” foams and their dependence on composition and processing have been studied (Shogren et at, 1998; Glenn et al., 2001; Shogren et al., 2002; Lawton et al., 2004), largely with the aim of improving mechanical properties and moisture resistance.
There is also growing interest in lignin-based materials. Lignin is an abundant renewable natural resource. A byproduct of paper manufacture, lignin is considered a fairly intractable waste material and is usually burned as fuel for lack of higher-value uses. Lignin is also produced as a byproduct in the refining process by which cellulose is isolated from lignocellulosic feedstocks. Starch-lignin materials can therefore be envisioned as becoming integrated into the production of bioethanol.
The properties and uses of lignin have been reviewed (Glasser et al., 2000; Hu, 2002). Kumar et al. (2009) have reviewed applications of lignin combined with other polymers. Baumberger (2002) has reviewed applications of lignin specifically in starch-lignin films.
Stevens et al. (2007) have examined thermoplastic starch-kraft lignin-glycerol blends prepared by film casting and extrusion. Stevens et al. (2010) prepared starch-lignin foams prepared with a technique similar to compression molding, whereby starch, water, and additives are heated in molds.
Starch-lignin foams have not previously been prepared by extrusion. The major applications for extruded starch-lignin foams are biodegradable packaging materials for single or short-term use, as alternatives to recalcitrant foamed polystyrene.
The known starch-lignin foam therefore possesses properties of interest. The process for production does not lend itself to continuous production streams, and the inhomogeneous product with a distinct outer layer represents characteristics subject to further investigation.
The major applications for starch-lignin foams would be packaging containers for single or short-term use, as biodegradable alternatives to foamed polystyrene.
Lignin is soluble in aqueous solution only at high pH. In studies of starch-lignin cast films (Stevens et A, 2007), ammonium hydroxide was used to raise the pH of the casting solution and was found to be a requirement for obtaining viable films. Preparing starch-lignin films by extrusion, on the other hand, had no significant high-pH requirement (Stevens et A, 2007). Lignin was found to have little effect on foam density. Stevens et al. proposed that extrusion may lead to lower densities in starch-lignin foams than foams obtained by compression molding, but without testing or developing a process to extrude the mixture.
SEM images of a starch foam and a starch lignin foam are shown in FIGS. 1A-1D. The features of starch foams (FIG. 1A) have been observed previously (Tiefenbacher, 1993; Shogren et al., 1998). Below a thin surface ‘skin’ of approximately 100 μma in thickness, there is a region of cellular structure containing 100-200 μm voids. The major internal region of the foam consists of large voids of up to 1 mm in size. The boundaries separating these regions are not sharp, but the combined thickness of the outer skin and smaller voids in the present micrographs is approximately 0.045 cm, similar to what has been observed previously (Tiefenbacher, 1993; Shogren et A, 1998). Starch-lignin foams display the same features (FIG. 1B). Lack of contrast makes the location of the dispersed lignin impossible. Nevertheless, the SEM images show that 20% lignin can be incorporated into starch foams without collapse of the foam and with no major change in morphology.
FIGS. 1C and 1D show enlarged images of the starch and starch lignin compression molded samples, respectively. The walls of the internal cells are approximately 10 μm thick, whether or not the foams contain lignin. Therefore, SEM indicates that replacing 20% of the starch with has no deleterious effect on overall morphology.
X-ray diffraction patterns of the starch and starch lignin compression molded samples are shown in FIG. 2. The significant diffraction maximum at 19.4° and a weaker maximum at 12.7° in both samples indicate the presence of residual structure of the V form of starch (Willett and Shogren, 2002; Shogren et al., 1998; Shogren and Jasberg, 1994). The absence of the B structure indicates that the native structure in the starch granule was destroyed during foam formation. Some of the amylose probably recrystallized into the V form during the cooling (Shogren and Jasberg, 1994).
Differential scanning calorimetry (DSC) shows that the starch foam displays, within the measured temperature range, a broad endothermic peak and a second smaller feature. Peak temperatures were 85±1° C. and 95±2° C., respectively. The integrated area, including both features and averaged over four specimens, corresponds to ΔH=2.0±0.1 J/g of dry starch. DSC features observed with starch samples depend on water content, age, source plant, and sample history (Shogren, 1992; Shogren and Jasberg, 1994; Maaruf et al., 2001). The thermal features in the starch foam indicate that heat treatment of starch during foam formation leaves some residual starch structure (Shogren and Jasberg, 1994). XRD analysis indicates that structure to be the V form of amylose. X-ray diffraction analysis indicates the presence of residual structure in both samples, but only the starch sample displays a thermal transition by DSC. This result indicates that, when lignin is present, starch-lignin interactions are sufficient to inhibit the thermal transition.
FIG. 3 shows a plot of in, versus t1/2 for a starch sample and starch-lignin sample, with mt in units of g/cm2 and t in seconds. The behavior is initially linear, but the slopes increase at longer times. The results of this empirical model indicate qualitatively that lignin impedes the absorption of water in the compression molded product. The ratio of the limiting slopes, at short times, is approximately 2:1, indicating a ratio of effective diffusion constants of approximately 4:1. The specimens were cut from the original larger samples, exposing voids along the edges, but for the sample sizes used here, only 10% of the surface area was exposed.
For the starch sample D=2.68·10−6 cm2/sec, and for the starch lignin sample D=0.80·10−6 cm2/sec. Lignin appears to impede diffusion into the outer layers of the foam but does not affect the diffusion mechanism. The ratio of the two effective diffusion constants is 3.4, indicating a significant improvement in water resistance in the starch-lignin foam. Baumberger et al. (1998), who studied starch-lignin films, also found that lignin improves water resistance, as long as no plasticizer is used. Stevens et al. (2007) found that if glycerol is used to plasticize starch-lignin films, the effect of the glycerol is to reduce or eliminate the hydrophobic effect of lignin.
The load deflection curves for the compression molded starch control samples showed an increase in strain beyond the point of maximum stress; they showed a yield. Beyond the yield, there was an additional strain of approximately 0.2% before the sample broke. On the other hand, foams containing lignin displayed no yield; they broke at the maximum measured stress. Shogren et al. (1998) and Lawton et al. (1999) have shown that starch content, plant source, and moisture content affect the mechanical properties of foams prepared by compression molding. In foams with 20% lignin (prepared with ammonium hydroxide), the ammonium hydroxide had the effect of significantly decreasing flexural strength (99% confidence level), but had no further effect on strain at maximum stress. The modulus of elasticity was larger than the value for the starch control (95% confidence level).